The present disclosure relates generally to methods of forming articles of semiconducting materials, and also to pre-treated semiconducting materials used in such methods.
Semiconducting materials are used in a variety of applications, and may be incorporated, for example, into electronic devices such as photovoltaic devices. Photovoltaic devices convert light radiation into electrical energy through the photovoltaic effect.
The properties of semiconducting materials depend on a variety of factors, including crystal structure, the concentration and type of intrinsic defects, and the presence and distribution of dopants and other impurities. Within a semiconducting material, the grain size and grain size distribution, for example, can impact the performance of resulting devices. By way of example, the electrical conductivity and thus the overall efficiency of a semiconductor-based device such as a photovoltaic cell will generally improve with larger and more uniform grains.
For silicon-based devices, silicon may be formed using a variety of techniques, which can be used to form a variety of shapes such as an ingot, sheet or ribbon. The silicon may be supported or unsupported by an underlying substrate. However, conventional methods for making supported and unsupported articles of silicon have a number of shortcomings.
Methods of making unsupported thin semiconducting material sheets, including silicon sheets, may be slow or wasteful of the semiconducting material feedstock. Unsupported single crystalline semiconducting materials can be produced, for example, using Czochralski or Bridgman processes. However, such bulk methods may disadvantageously result in significant kerf loss when the material is cut into thin sheets or wafers. Additional methods by which unsupported polycrystalline semiconducting materials can be produced include electromagnetic casting and direct net-shape sheet growth methods such as ribbon growth processes. These techniques tend to be slow and expensive. Polycrystalline silicon ribbon produced using silicon ribbon growth technologies is typically formed at a rate of only about 1-2 cm/min.
Supported semiconducting material sheets may be produced less expensively, but the semiconducting material sheet may be limited by the substrate on which it is formed, and the substrate may have to meet various process and application requirements, which may be conflicting.
Example methods for producing unsupported polycrystalline semiconducting materials using an exocasting process are disclosed in U.S. patent application Ser. No. 12/394,608, filed Feb. 27, 2009, U.S. patent application Ser. No. 12/466,143, filed May 14, 2009, and U.S. patent application Ser. No. 12/632,837, filed Dec. 8, 2009, the disclosures of which are hereby incorporated by reference.
As described herein, the inventors have now discovered additional methods by which supported and unsupported articles of semiconducting materials may be made. The disclosed methods may facilitate formation of semiconducting materials (e.g., exocast materials) having desirable attributes such as uniform thickness and a desired microstructure while reducing material waste and increasing the rate of production.